Monolithic, galleryless piston and method of construction thereof

ABSTRACT

A galleryless piston for an internal combustion engine is provided. The piston has a monolithic piston body including an upper wall forming an upper combustion surface with first and second portions. The first portion extends annularly along an outer periphery of the upper wall and the second portion includes a combustion bowl. The first portion can also include valve pockets formed therein to reduce weight. The upper wall has an undercrown surface directly opposite the second portion of the upper combustion surface. To enhance cooling, a center portion of the undercrown surface is concave, such that oil is channeled during reciprocation of the piston from one side to the opposite side of the piston.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. Continuation Patent Application claims the benefit of U.S.Utility patent application Ser. No. 14/535,839, filed Nov. 7, 2014,which claims the benefit of U.S. Provisional Application No. 61/901,287,filed Nov. 7, 2013, and the benefit of U.S. Provisional Application No.62/011,876, filed Jun. 13, 2014, which are each incorporated herein, byreference, in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to internal combustion engines, andmore particularly to pistons therefor.

2. Related Art

Engine manufacturers are encountering increasing demands to improveengine efficiencies and performance, including, but not limited to,improving fuel economy, reducing oil consumption, improving fuelsystems, increasing compression loads and operating temperatures withinthe cylinder bores, reducing heat loss through the piston, improvinglubrication of component parts, decreasing engine weight and makingengines more compact, while at the same time decreasing the costsassociated with manufacture. While desirable to increase the compressionload and operation temperature within the combustion chamber, it remainsnecessary to maintain the temperature of the piston within workablelimits. Accordingly, although desirable to increase the compression loadand operation temperature within the combustion chamber, achieving thisgoal comes with a tradeoff in that these desirable “increases” limit thedegree to which the piston compression height, and thus, overall pistonsize and mass can be decreased. This is particularly troublesome withtypical piston constructions having a closed or partially closed coolinggallery to reduce the operating temperature of the piston. Further, thecost to manufacture pistons having upper and lower parts joined togetheralong a bond joint to form the closed or partially closed coolinggallery is generally increased due to the joining process used to bondthe upper and lower parts together. Further, the degree to which theengine weight can be reduced is impacted by the need to make theaforementioned “cooling gallery-containing” pistons from steel so theycan withstand the increase in mechanical and thermal loads imposed onthe piston.

SUMMARY OF THE INVENTION

A piston constructed in accordance with this invention overcomes theaforementioned disadvantages of known piston constructions and otherdisadvantages, as will become apparent to those skilled in the art uponreading the disclosure and viewing the drawings herein.

The piston is constructed of a monolithic piece of steel, therebyproviding the piston with enhanced strength and durability to withstandincreased compression loads and temperatures within a cylinder bore,such as those loads and temperatures seen in modern high performanceengines. Further, due to the novel single piece configuration of thepiston, the costs associated with manufacture of the piston are reducedin that the typical processes used to join piston upper and lower partstogether are not employed. Further, in a piston constructed inaccordance with the invention, the compression height (CH) and weight ofthe piston are able to be significantly reduced relative to a pistonincluding a closed or partially closed cooling gallery, particularlysince the piston does away with the presence of a floor commonly used toform a cooling gallery, thereby allowing an engine in which the pistonsare deployed to be made more compact, lightweight and fuel efficient.

In accordance with one aspect of the invention, a galleryless piston foran internal combustion engine is provided. The piston has a monolithicpiston body extending along a central longitudinal axis with an upperwall forming an upper combustion surface and an annular ring belt regiondepending from the upper combustion surface. The upper combustionsurface has first and second portions, wherein the first portion extendsannularly along an outer periphery of the upper wall and the secondportion forms a combustion bowl depending radially inwardly from thefirst portion. The upper wall also has an undercrown surface formed onan underside thereof. The undercrown surface is located directlyopposite the second portion of the upper combustion surface. The pistonbody also includes a pair of skirt portions depending from the ring beltregion, and a pair of pin bosses spaced from one another by the skirtportions and providing a pair of laterally spaced pin bores. The pistonbody is free of a cooling gallery along the undercrown surface betweenthe skirt portions and pin bosses. Thus, the undercrown surfacecomprises an expansive area for contact by cooling oil while the pistonis in use. To further enhance cooling of the piston, the undercrownsurface includes a concave center portion located along the centrallongitudinal axis which channels oil from one side of the piston to theopposite side.

In accordance with another aspect of the invention, a method ofconstructing a galleryless piston for an internal combustion engine isprovided. The method includes forming a monolithic piston body by atleast one of a machining, forging and casting process. The piston bodyextends along a central longitudinal axis and has an upper wall formingan upper combustion surface with an annular ring belt region dependingfrom the upper combustion surface. The upper combustion surface hasfirst and second portions, the first portion extending annularly alongan outer periphery of the upper wall and the second portion forming acombustion bowl depending radially inwardly from the first portion. Theupper wall also has an undercrown surface formed on an undersidethereof. The undercrown surface is located directly opposite the secondportion of the upper combustion surface. The piston body also includes apair of skirt portions depending from the ring belt region, and a pairof pin bosses spaced from one another by the skirt portions andproviding a pair of laterally spaced pin bores. The piston body is freeof a cooling gallery along the undercrown surface between the skirtportions and pin bosses, and the undercrown surface includes a concavecenter portion located along the central longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the invention willbecome more readily appreciated when considered in connection with thefollowing detailed description of presently preferred embodiments andbest mode, appended claims and accompanying drawings, in which:

FIG. 1 is a bottom perspective view of a piston constructed inaccordance with an embodiment of the invention;

FIG. 2 is a cross-sectional view taken generally transversely to a pinbore axis of a piston in accordance with an embodiment of the invention;and

FIG. 3 is a bottom perspective view of a piston constructed inaccordance with another embodiment of the invention.

DETAILED DESCRIPTION

Referring in more detail to the drawings, FIGS. 1-3 illustrate views ofa piston 10 constructed in accordance with example embodiments of theinvention for reciprocating movement in a cylinder bore or chamber (notshown) of an internal combustion engine, such as a modern, compact, highperformance vehicle engine, for example. The piston 10 is constructedhaving a monolithic body formed from a single piece of material, such asvia machining, forging or casting, with possible finish machiningperformed thereafter, if desired, to complete construction. Accordingly,the piston 10 does not have a plurality of parts joined together, suchas upper and lower parts joined to one another, which is commonplacewith pistons having enclosed or partially enclosed cooling galleriesbounded or partially bounded by a cooling gallery floor. To thecontrary, the piston 10 is “galleryless” in that it does not have acooling gallery floor or other features bounding or partially bounding acooling gallery. The piston body, being made of steel, is strong anddurable to meet the high performance demands, i.e. increased temperatureand compression loads, of modern day high performance internalcombustion engines. The steel (i.e., the steel alloy) used to constructthe body can be SAE 4140 grade or different, depending on therequirements of the piston 10 in the particular engine application. Dueto the piston 10 being galleryless, and the novel configuration of thebody, among other things discussed below, minimizes the weight andcompression height (CH) of the piston 10, thereby allowing an engine inwhich the pistons 10 are deployed to achieve a reduced weight and to bemade more compact. Further yet, even though being galleryless, the novelconstruction discussed below and shown in the Figures allows the piston10 to be sufficiently cooled during use to withstand the most severeoperating temperatures.

The piston body has an upper head or top section providing a top wall,also referred to as upper wall 14, which provides an upper combustionsurface 16 that is directly exposed to combustion gasses within thecylinder bore of the internal combustion engine. The upper combustionsurface 16 includes an annular first portion 18 formed as asubstantially planar surface extending along an outer periphery of theupper wall 14 and a second portion 20 forming a combustion bowl. Thesecond portion 20 of the upper combustion surface 16, which forms thecombustion bowl, has a non-planar, concave, or undulating surface thatdepends from the planar first portion 18. An undercrown surface 24 isformed on an underside of the upper wall 14, directly opposite thesecond portion 20 of the upper combustion surface 16. The undercrown 24is defined here to be the surface that is visible, excluding the pinbores 40, when observing the piston 10 straight on from the bottom. Thesecond portion 20 of the upper wall 14 has a thickness t extending fromthe upper combustion surface 16 to the underside of the upper wall 14.The portion of the underside of the upper wall 14 which is considered tobe the undercrown surface 24 is a portion that extends to a distanceaway from the second portion 20 of the upper combustion surface 16 nomore than two times the minimum thickness t_(min) of the upper wall 14along the second portion 20. Accordingly, the undercrown surface 24 isgenerally form fitting to the combustion bowl of the upper combustionsurface 16. The undercrown surface 14 is located at a minimum distancefrom the combustion bowl and is substantially the surface on theopposite side from the combustion bowl. The undercrown surface 24 isopenly exposed, as viewed from an underside of the piston 10, and it isnot bounded by an enclosed or partially enclosed cooling gallery, or anyother features tending to retain oil or a cooling fluid near theundercrown surface.

The annular first portion 18 of the upper wall 14 forms an outerperiphery of the upper wall 14 and surrounds the second portion formingthe combustion bowl, which depends therefrom. Thus, the second portion20, including the combustion bowl, is recessed below the uppermost firstportion 18 of the upper combustion surface 16. In the exampleembodiments, at least one valve pocket 29 having a curved profile isformed in the annular first portion 18 of the upper wall 14.

In the example embodiment, the combustion bowl of the second portion 20of the upper wall 14 extends continuously through a central axis 30 andacross the entire diameter of the piston 10, between opposite sides ofthe annular first portion 18. The combustion bowl can comprise a concavesurface extending continuously between the opposite sides of the annularfirst portion 18. Alternatively, the combustion bowl wall can becontoured, for example to provide an upper apex, also referred to ascenter peak (not shown), which may lie coaxially along the central axis30 of the piston 10, or may be radially offset relative to the pistoncentral axis 30. The top section of the piston 10 further includes aring belt region 32 that depends from the upper combustion surface 16 toprovide one or more ring grooves 34 for receipt of one or morecorresponding piston rings (not shown).

The piston body further includes a bottom section including a pair ofpin bosses 38 depending generally from the upper wall 14. The pin bosses38 each have a pin bore 40, preferably bushingless given the steelconstruction, wherein the pin bores 40 are laterally spaced from oneanother coaxially along a pin bore axis 42 that extends generallytransversely to the central longitudinal axis 30. The pin bosses 38 havegenerally flat, radially outermost surfaces, referred to as outer faces43, that are spaced from one another along the pin bore axis 40 adistance PB, shown as being generally parallel with one another. The PBdimension is minimized, thereby maximizing an exposed area of arecessed, generally cup-shaped region, referred to hereafter asundercrown pockets 50. The undercrown pockets 50 are located radiallyoutwardly of the pin bosses 38 and form a portion of the undercrownsurface 24. The undercrown pockets 50 also extend radially outwardlybeyond the undercrown surface 24 along an underside surface of theannular first portion 18 of the upper combustion surface 16 and dependfrom the upper wall 14 along an inner surface of the ring belt region32. As such, with the 2-dimensinional and 3-dimensional surface area ofthe pockets 50 being maximized, at least in part due to the minimizeddistance PB, the cooling caused by oil splashing or being sprayedupwardly from the crankcase against the exposed surface of theundercrown pockets 50 is enhanced, thereby lending to further cooling ofthe upper combustion surface 16, the undercrown 24, as well as a portionof the ring belt region 34.

The pin bores 40 each have a concave uppermost load bearing surface,referred to hereafter as uppermost surface 44, disposed near the ringbelt region 32. As such, the compression height CH is minimized (thecompressing height is the dimension extending from the pin bore axis 42to the upper combustion surface 16). The pin bosses 38 are joined viaouter panels, also referred to as struts 46, to diametrically oppositeskirt portions, also referred to as skirt panels 48.

The pin bosses 38, skirt panels 48 and struts 46 bound an open regionextending from a lowermost or bottom surface 51 of the struts 46 andskirt panels 48 to the undercrown surface 24. In the example embodimentof FIGS. 1 and 2, there are no ribs located along the undercrown surface24, along the pin bosses 38, along the skirt panels 48, or along thestruts 46 in the open region. In addition, no closed or partially closedcooling gallery is formed in the open region. However, the piston 10 caninclude a stepped region 54 along the uppermost edge of each skirt panel48 adjacent the undercrown surface 24. The stepped regions 54 provideadditional material so that the ring belt region 32 can be formed in theupper part of the piston 10. In the example embodiment, the steppedregions 54 are not considered part of the undercrown surface 24. Inanother embodiment, for example the embodiment shown in FIG. 3, thepiston 10 does include a pair of ribs 58 along the undercrown surface 24to enhance cooling. The ribs 58 extend continuously along the undercrownsurface 24 between the opposite skirt panels 38.

The open region along the underside of the piston 10 provides directaccess to oil splashing or being sprayed from within the crankcasedirectly onto the undercrown surface 24, thereby allowing the entireundercrown surface 24 to be splashed directly by oil from within thecrankcase, while also allowing the oil to freely splash about the wristpin (not shown), and further, significantly reduce the weight of thepiston 10. Accordingly, although not having a typical closed orpartially closed cooling gallery, the generally open configuration ofthe galleryless piston 10 allows optimal cooling of the undercrownsurface 24 and lubrication to the wrist pin joint within the pin bores40, while at the same time reducing oil residence time on the surfacesnear the combustion bowl, which is the time in which a volume of oilremains on the surface. The reduced residence time can reduce unwantedbuild-up of coked oil, such as can occur in pistons having a closed orsubstantially closed cooling gallery. As such, the piston 10 remains“clean” over extended use, thereby allowing it to remain substantiallyfree of build-up.

Owing to the optimal cooling of the undercrown surface 24 is thepercentage of the undercrown surface 24 directly underlying the uppercombustion surface 16 that is directly exposed to the splashing andsprayed oil from the crankcase. The undercrown surface 24 of the piston10 has greater a total surface area (3-dimensional area following thecontour of the surface) and a greater projected surface area(2-dimensional area, planar, as seen in plan view) than comparativepistons having a closed or partially closed cooling gallery.

In one embodiment, the undercrown surface 24 has a total exposed surfacearea, defined as the 3-dimensional area following the contour of theundercrown surface 24 that is greater than 40 percent of, and betweenabout 40-90 percent of the area of the cylinder bore, and thus, betweenabout 40-90 percent of the 2-dimensional area defined by the maximumouter diameter OD of the piston 10 (i.e., 40% of πD²/4, where D is themaximum outer diameter OD). The undercrown surface 24 can also have aprojected surface area, defined as the 2-dimensional area seen lookinggenerally along the central longitudinal axis 30 from the bottom of thepiston 10 between about 35-60 percent of the area of the cylinder bore,and thus, between about 35-60 percent of the area defined by the maximumouter diameter OD of the piston 10. Further yet, the exposed area of theundercrown surface 24 can have a diameter D between about 75-90 percentof the diameter of the cylinder bore, and thus, between about 75-90percent of the maximum outer diameter OD of the piston 10. Further yet,the exposed area of the undercrown surface 24 can have a diameter Dbetween about 85-140 percent of the diameter CD of the combustion bowl,which is in contrast to a maximum of 100 percent for a piston having aclosed or substantially closed cooling gallery. However, the percentagesof relative surface areas and relative diameters can vary from theranges disclosed above while still provide for enhanced cooling. Thepercentages of relative surface areas and relative diameters of theexposed undercrown surface 24 of the piston 10 are far in excess ofconventional pistons, and in some cases, are upwards to 3 times greateror more. As such, the upper combustion surface 16 can be cooled directlyvia oil splashing upwardly from the crankcase, which can be coupled withthe assistance from oil jets, if desired.

As mentioned above, the undercrown pockets 50 of the piston 10 define atleast a portion of the undercrown surface 24, as well as a portion of anunderside of the first portion 18 and a portion of an inner surface ofthe annular ring belt region 32. In one embodiment, the undercrownpockets 50 have a total 3-dimensional surface area between about 45-85percent of the maximum 2-dimensional area of the piston 10. However, the2-dimensional and 3-dimensional surface area of the undercrown pockets50 can vary from the ranges disclosed above while still being able tocontribute significantly to the cooling of the regions of the uppercombustion surface 16 located directly above the pockets 50.

In accordance with another aspect of the invention, at least a centerportion 52 of the undercrown surface 24 of the piston 10 disposedbetween the opposite skirt panels 38 and the opposite pin bosses 38 isconcave in form, when viewing from the bottom of the piston 10. As such,oil is channeled during reciprocation of the piston 10 from one side ofthe piston 10 to the opposite side of the piston 10, thereby acting tofurther enhance cooling of the piston 10. This concave center portion 52has a length L extending longitudinally between the skirt panels 38 anda width W extending between the pin bosses 38. The length L of theconcave center portion 52 is typically greater than the width W.

In the example embodiment shown in FIG. 2, the concave center portion 52is axially offset from the surrounding area 56 of the undercrown surface24. For example, the concave center portion 52 can be disposed closer tothe pin bosses 38 than the surrounding area 56. In the exampleembodiment of FIG. 3, the concave center portion 52 extendslongitudinally between the opposite skirt panels 38, and the ribs 58 aredisposed on opposite sides of the concave center portion 52. In otherwords, the ribs 58 separate a least a portion of the concave centerportion 52 from the pin bosses 38.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is, therefore, to be understood thatthe invention may be practiced otherwise than as specifically described,and that the scope of the invention is defined by any ultimately allowedclaims.

What is claimed is:
 1. A piston for an internal combustion engine,comprising: a monolithic piston body extending along a centrallongitudinal axis; said piston body having an upper wall forming anupper combustion surface with an annular ring belt region depending fromsaid upper combustion surface; said upper combustion surface havingfirst and second portions, said first portion extending annularly alongan outer periphery of said upper wall and said second portion forming acombustion bowl depending radially inwardly from said first portion;said upper wall having an undercrown surface formed on an undersidethereof, said undercrown surface being located directly opposite saidsecond portion of said upper combustion surface; said piston bodyincluding a pair of skirt portions depending from said ring belt region;said piston body including a pair of pin bosses spaced from one anotherby said skirt portions and providing a pair of laterally spaced pinbores; said piston body being free of a cooling gallery along saidundercrown surface between said skirt portions and pin bosses; and saidundercrown surface including a concave center portion located along saidcentral longitudinal axis.
 2. The piston of claim 1, wherein saidconcave center portion has a length extending longitudinally betweensaid skirt panels and a width extending between said pin bosses, whereinthe length of said concave center portion is greater than the width. 3.The piston of claim 1, wherein said concave center portion is axiallyoffset from a surrounding area of said undercrown surface.
 4. The pistonof claim 3, wherein said concave center portion is disposed closer tosaid pin bosses than said surrounding area.
 5. The piston of claim 1,wherein at least one valve pocket having a curved profile is formed insaid first portion of said upper wall.
 6. The piston of claim 1, whereinsaid piston body includes a stepped region along an uppermost edge ofeach of said skirt panels adjacent said undercrown surface.
 7. Thepiston of claim 6, wherein said stepped regions do not form a portion ofsaid undercrown surface.
 8. The piston of claim 1, wherein an undercrownpocket is disposed radially outwardly of each of said pin bosses, andsaid undercrown pockets form a portion of said undercrown surface. 9.The piston of claim 1, wherein said piston body is free of ribs alongsaid undercrown surface, said skirt panels, and said pin bosses.
 10. Thepiston of claim 1, wherein said piston body presents a thicknessextending from said upper combustion surface to the underside of saidupper wall, said undercrown surface is a portion of said underside whichextends to a distance away from the second portion of said uppercombustion surface, wherein the distance is no more than 2 times theminimum thickness along said second portion.
 11. The piston of claim 1,wherein said piston body is formed of steel.
 12. The piston of claim 1,wherein said concave center portion of said undercrown surface has alength extending longitudinally between said skirt panels and a widthextending between said pin bosses, wherein the length of said concavecenter portion is greater than the width; said concave center portion isaxially offset from a surrounding area of said undercrown surface; saidconcave center portion is disposed closer to said pin bosses than saidsurrounding area; at least one valve pocket having a curved profile isformed in said first portion of said upper wall; said piston bodyincludes a stepped region along an uppermost edge of each of said skirtpanels adjacent said undercrown surface, and said stepped regions do notform a portion of said undercrown surface; said piston body includes anundercrown pocket disposed radially outwardly of each of said pinbosses, and said undercrown pockets form a portion of said undercrownsurface; said piston body is free of ribs along said undercrown surface,said skirt panels, and said pin bosses; said piston body presents athickness extending from said upper combustion surface to an undersideof said upper wall, said undercrown surface is a portion of saidunderside which extends to a distance away from said second portion ofsaid upper combustion surface, and said distance is no more than 2 timesthe minimum thickness along said second portion; and said piston body isformed of steel.
 13. A piston for an internal combustion engine,comprising: a monolithic piston body extending along a centrallongitudinal axis; said piston body having an upper wall forming anupper combustion surface with an annular ring belt region depending fromsaid upper combustion surface; said upper combustion surface havingfirst and second portions, said first portion extending annularly alongan outer periphery of said upper wall and said second portion forming acombustion bowl depending radially inwardly from said first portion;said upper wall having an undercrown surface formed on an undersidethereof, said undercrown surface being located directly opposite saidsecond portion of said upper combustion surface; said piston bodyincluding a pair of skirt portions depending from said ring belt region;said piston body including a pair of pin bosses spaced from one anotherby said skirt portions and providing a pair of laterally spaced pinbores; said piston body being free of a cooling gallery along saidundercrown surface between said skirt portions and pin bosses; and saidundercrown surface including a concave center portion located betweensaid pin bosses.
 14. A method of constructing piston, comprising thesteps of: forming a monolithic piston body by at least one of amachining, forging and casting process, the piston body extending alonga central longitudinal axis and having an upper wall forming an uppercombustion surface with an annular ring belt region depending from theupper combustion surface, the upper combustion surface having first andsecond portions, the first portion extending annularly along an outerperiphery of the upper wall and the second portion forming a combustionbowl depending radially inwardly from the first portion, the upper wallhaving an undercrown surface formed on an underside thereof, theundercrown surface being located directly opposite the second portion ofthe upper combustion surface, the piston body including a pair of skirtportions depending from the ring belt region, the piston body includinga pair of pin bosses spaced from one another by the skirt portions andproviding a pair of laterally spaced pin bores, the piston body beingfree of a cooling gallery along the undercrown surface between the skirtportions and pin bosses, and the undercrown surface including a concavecenter portion located along the central longitudinal axis.
 15. Themethod of claim 14, wherein the forming step includes machining theconcave center portion of the undercrown surface after casting thepiston body.
 16. The method of calm 15, wherein the concave centerportion has a length extending longitudinally between the skirt panelsand a width extending between the pin bosses, and wherein the step ofmachining the concave center portion includes forming the length of theconcave center portion greater than the width.
 17. The method of claim14, wherein the step of machining the concave center portion includesforming the concave center portion axially offset from a surroundingarea of the undercrown surface.
 18. The method of claim 14, wherein theforming step includes forming at least one valve pocket having a curvedprofile in the first portion of the upper wall.
 19. The method of claim14, wherein the forming step includes a stepped region along anuppermost edge of each of the skirt panels adjacent the undercrownsurface.
 20. The method of claim 14, wherein the forming step includesforming undercrown pockets radially outwardly of the pin bosses, whereinthe undercrown pockets form a portion of the undercrown surface.
 21. Themethod of claim 14, wherein the concave center portion of the undercrownsurface has a length extending longitudinally between the skirt panelsand a width extending between the pin bosses, wherein the length of theconcave center portion is greater than the width, the concave centerportion is axially offset from a surrounding area of the undercrownsurface, and the concave center portion is disposed closer to the pinbosses than the surrounding area; wherein the piston body providedduring the forming step is formed of steel and further includes at leastone valve pocket having a curved profile in the first portion of theupper wall, a stepped region along an uppermost edge of each of theskirt panels adjacent the undercrown surface which does not form aportion of the undercrown surface, and an undercrown pocket disposedradially outwardly of each of the pin bosses which forms a portion ofthe undercrown surface; wherein the piston body provided during theforming step is free of ribs along the undercrown surface, the skirtpanels, and the pin bosses; and wherein the piston body provided duringthe forming step presents a thickness extending from the uppercombustion surface to an underside of the upper wall, the undercrownsurface is a portion of the underside which extends to a distance awayfrom the second portion of the upper combustion surface, and thedistance no more than two times the minimum thickness along the secondportion.
 22. A method of constructing piston, comprising the steps of:forming a monolithic piston body by at least one of a machining, forgingand casting process, the piston body extending along a centrallongitudinal axis and having an upper wall forming an upper combustionsurface with an annular ring belt region depending from the uppercombustion surface, the upper combustion surface having first and secondportions, the first portion extending annularly along an outer peripheryof the upper wall and the second portion forming a combustion bowldepending radially inwardly from the first portion, the upper wallhaving an undercrown surface formed on an underside thereof, theundercrown surface being located directly opposite the second portion ofthe upper combustion surface, the piston body including a pair of skirtportions depending from the ring belt region, the piston body includinga pair of pin bosses spaced from one another by the skirt portions andproviding a pair of laterally spaced pin bores, the piston body beingfree of a cooling gallery along the undercrown surface between the skirtportions and pin bosses, and the undercrown surface including a concavecenter portion located between said pin bosses.